The invention relates to composite films having electrically switchable optical properties comprising light-scattering support materials based on electrophoretically mobile particles in a suspension liquid.
Information systems, such as, for example, signs, advertising boards, price indicators, timetable displays, computer displays or flat-panel display screens in general are used to display text, symbols or graphics. They should have high contrast, even in frontal lighting, be legible even at acute angles and have adequate inherent luminosity or have corresponding external illumination. The information displayed may be fixed, for example advertising hoardings, or electronically changeable, for example computer displays.
Many of these information systems have no inherent luminosity and are lit externally, for example by normal daylight or room light, front-lighting frequently being preferred owing to the reflection-free illumination.
An application of information systems which is of particularly outstanding commercial importance is in flat-panel display screens, as employed, for example, in portable computers. Flat-panel display screens are produced either using self-illuminating displays, which do not require an illumination system, or using non-self-illuminating displays, which operate, for example, on the basis of liquid crystals or electrophoretic systems. Non-self-illuminating displays are, in simplified terms, constructed from at least two layers: an illumination unit and a layer on which the electronically changeable information can be displayed, referred to here as the visualization layer. The illumination unit can be used as back- or front-lighting. The type of illumination unit is selected depending on the transparency and/or reflectivity of the visualization layer.
The illumination unit must ensure the highest possible contrast for good lighting of the entire field of view. This can frequently only be achieved by means of high-power illumination units. However, the provision of the energy necessary for the latter requires corresponding high-power batteries, which are currently still associated with a considerable increase in the overall weight.
Illumination units of modern computer displays frequently consume over 90% of the energy to be used for the entire screen. In the back-lighting systems for liquid-crystal displays (LCDs), a considerable proportion of the light generated is filtered out by the polarization layers for example and is thus not available for the illumination.
In many cases, flat lamps or a multiplicity of lamps with corresponding light diffuser screens or meshes are employed for back-lighting systems of liquid-crystal displays (LCDs); other systems are based on an illumination unit with lateral introduction of the light into a waveguide plate and corresponding reflection units on the underside or excitable output points on the upper side of the waveguide plate. These techniques can only be used for back-lighting systems and not for front-lighting systems arranged between the visualization layer and the observer, since the emitted light is emitted both in the direction of the observer and toward the visualization layer, and it is therefore difficult, if at all possible, to distinguish the displayed information on the visualization layer.
Other display techniques use flat and thin electroluminescent lamps or small fluorescent units with a diffuser device. Although the electroluminescent lamps consume less energy than the fluorescent back-lighting systems, they are not as powerful and usually do not emit over the entire light spectrum necessary for the operation of color display screens. In addition, the life of electroluminescent lamps is unsatisfactory.
A novel development for the display of electronically changeable information is the xe2x80x9celectronic inkxe2x80x9d of Prof. J. Jacobson et al. This technique utilizes the alignment of single- or multicolor pigment particles in an electric field for the display of image information. Details can be obtained, for example, from J. Jacobson et al., IBM System Journal 36 (1997), pages 457-463, or B. Comiskey et al., Nature, Vol. 394, July 1998, pages 253-255.
For the preparation of corresponding bipolar, single- or two-color particles in various embodiments and their use in electrophoretic displays, reference is made, for example, to WO 98/03896, which describes how these particles are suspended in an inert liquid and encapsulated in small bubbles of a support material. This technique allows the macroscopic display of two colors by rotation of a two-color particle, depending on the applied electric field.
WO 98/19208 describes a similar electrophoretic display in which electrophoretically mobile particles in an optionally colored liquid can be moved within a microcapsule by an electric field. Depending on the field direction, the particles align to form an electrode and thus display macroscopically yes/no color information (either the color of the particles is visible or the color of the liquid is visible).
WO 98/41899 discloses electrophoretic displays which, although based on the principles described above, contain neither fluorescent nor reflective particles. In addition, the use of a suspension having liquid-crystalline behavior is also described. The liquid crystals block or facilitate electrophoretic migration of the particles, depending on the applied electric field.
WO 98/41898 likewise describes an electrophoretic display system of this type which can be produced, owing to its special arrangement, by a printing process, in particular by an ink-jet printing method. Both the electrodes and the electrophoretic display per se can advantageously be produced in successive printing steps.
It is a common feature of these techniques that the suspension liquid and the particles are embedded in capsules, bubbles or other cavities of a polymeric material. The particles can also be encapsulated with the suspension liquid; these capsules can then either be introduced in prefabricated form into the support material polymerization process or formed together with the support material in a complex emulsion polymerization. In neither case is the size or arrangement of the capsules or cavities uniform. Both the size and the two- or three-dimensional distribution of the microcapsules or cavities in the support material are subject to scatter which is difficult to control, firstly resulting in an inhomogeneous image information and secondly no making it difficult to achieve high contrast.
Systems of this type are, in particular, unsuitable for back-lighting since, due to their design, they are virtually opaque. In the case of simple illumination with visible incident light (front-lighting), the contrast is frequently unsatisfactory. Furthermore, the use of incident-light systems, i.e. with an external light source with visible light, makes it difficult to achieve uniformity of the illumination at the same time as continued good contrast.
The object of the present invention was to develop electrophoretic systems for the display of colors or information which have high luminosity and/or contrast at the same time as a flat design. The system should be thin and flexible in order to be able to be applied additionally to three-dimensional objects.
It has been found that a display system which uses electrophoretically mobile particles in cavities of a light-scattering microcompartment film has particularly high luminosity.
The present invention therefore relates to composite films having electrically switchable optical properties which are built up from two control electrodes and a microcompartment film having cavities containing electrophoretically mobile particles in a suspension liquid, where the microcompartment film consists of a light-scattering material.